1. Field of the Invention
The subject matter of this invention relates generally to apparatus for electrically detecting the eccentricity of a shaft and more specifically to apparatus for periodically sampling the eccentricity of a shaft which is turning at relatively low speeds to electrically determine the absolute value of eccentricity.
2. Description of the Prior Art
It is known that shafts such as steam turbine shafts have a tendency to buckle under their own weight when maintained in one angular position for a certain period of time. The buckling of the shaft tends to introduce eccentricity or wobble into the shaft when it is thereafter rotated. This means that the actual center of rotation of the shaft no longer corresponds to the geometric center of the shaft but is offset therefrom by some amount which is equal to eccentricity. A shaft which is eccentric when rotating may cause bearing damage, among other things. It is known that large shafts may be rotated in a speed range from approximately 1.5 revolutions per minute to 600 revolutions per minute during a start-up operation in which range eccentricity may be determined. Normally the shafts rotate at operational speeds of 1800 RPM or 3600 RPM to produce 60 hertz electrical power, or 1500 RPM or 3000 RPM to produce 50 hertz electrical power. However, an eccentric shaft at the previously mentioned operational speeds would tend to be destructive. One problem associated with attempting to measure eccentricity in the relatively low speed range, i.e., 1.5 RPM to 600 RPM, is the wide ratio of speeds which must be accommodated by the electronic equipment which is utilized to measure eccentricity. In the latter case, the speed ratio is 400 to 1 (600 to 1.5). Another problem is associated with utilizing electronic equipment to make measurements at the very low end of the frequency scale, i.e., 0.025 hertz (or 1.5 RPM). It would be advantageous, therefore, if an electronic eccentricity determining apparatus could be provided which would be operational and reliable over a wide range of frequencies or speeds of shaft rotation and which would also be reliably operational at the turning gear speed (1.5 RPM) of a turbine system. Some of the problems associated with the latter situations include the difficulty in coupling signals at the frequency of .025 hertz between the stages of electronic apparatus and the large amount of charging current which would be necessary for certain capacitive elements because of the wide range of the operational speed, i.e., 400 to 1.